Write long characteristics

Team, 

I am using nrf52810, sdk 17, sd112. 

I have setup a write characteristics, to receive some long data ( 64 bytes)

// write char
memset(&add_char_params, 0, sizeof(add_char_params));
add_char_params.uuid = LBS_UUID_WR_CHAR;
add_char_params.uuid_type = p_lbs->uuid_type;
add_char_params.init_len = 64;
add_char_params.max_len = 64;
add_char_params.char_props.read = 0;
add_char_params.char_props.write = 1;
add_char_params.is_value_user = true;

add_char_params.read_access = SEC_OPEN;
add_char_params.write_access = SEC_OPEN;

characteristic_add(p_lbs->service_handle, &add_char_params, &p_lbs->wr_char_handles);

I have set my MTU to 70

When I use NRF connect android app and issue small data (8-12 bytes), it arrives on the nrf52810. 

When I send long data anything longer than 25 bytes, it does not arrive. 

My questions are: 

1. Where is the memory for holding incoming characteristics data? 

does this flag control its location?  add_char_params.is_value_user = true;  the documentation reads :   /**< Indicate if the content of the characteristic is to be stored in the application (user) or in the stack.*/

In some examples I have seen this  attr_md.vloc = BLE_GATTS_VLOC_STACK;

how are the two related?

2. What is the purpose of  sd_ble_user_mem_reply

The documentation reads: 

3. who initiates the MTU exchange? is it sufficient to set the MTU on the NRF52 to 70 bytes? Does the android nrf connect request a larger MTU? 

Any step by step guidance on how to receive large data would be much appreciated. 

Regards, 

Hari 

NOTE: I WAS ABLE TO GET THIS WORKING

Upon connection from android app, I made the nrf52 request mtu of 70 to android app. After that transfer is happening properly. 

Please confirm I have done the right thing.